1. Field of the Invention
The present invention relates to an image output device suitable for use with a digital copier, printer, facsimile or electronic file.
2. Description of the Prior Art
Prior art printers which simultaneously print a plurality of pixels include LED printers which form latent images through an electrophotographing method by turning on and off light emitting diodes (LED's) in LED lines, and thermal printers which have lines of heat generating elements contacted to a thermal paper and form images by controlling energization of the heat generating elements.
In order to simultaneously form one line of an image at a high density, a number of dot forming elements such as LED's must be provided. For example, when an image is to be formed on size A3 record paper by pixels of 16 dots/mm, 4800 dot forming elements are required because the length of the record paper is approximately 300 mm. In order to supply control signals for controlling the formation or non-formation of the dots by the 4800 dot forming elements, 4800 signal lines should be taken out and each of the signal lines should be controlled, which is very difficult to attain.
In another approach, a memory which serially receives an image data from an external unit, temporarily stores therein at least one line of data to be recorded on a record paper and outputs the data in parallel. A driver for driving the dot forming elements in accordance with the output data is integrated and the integrated circuit is mounted near the dot forming element array. Output terminals of the integrated driver and the dot forming elements are coupled by wire bonding. A head including the dot forming elements having such a control circuit mounted thereon is usually used.
In this method, only binary recording is achieved because only a binary signal that is, ON an or OFF signal is applied to each of the dot forming elements as the control signal.
On the other hand, in a method in which a duty period of the voltage or current supplied to the dot forming elements is changed to change tonality, shift registers, each having as many bits per pixel as the number of tonalities required, a data transfer function, a timer function and a recirculation function are cascade-connected, one for each of the dot forming element lines. In this method, the image data which represents the tonality by the number of "1" bits in a predetermined number of bits are serially and sequentially supplied to the shift register, one line of the data on the record paper at a time, and then the shift register is operated in the recirculation mode to form the respective dots.
When the device which uses this method is used as an output device of a facsimile, the time required for the serial-parallel conversion in the shift register does not raise a significant problem because the density data is also serially transmitted and a long time is required to transmit one frame of data.
However, a problem occurs when an image is to be reproduced at a high speed. For example, when the image is to be formed on size A3 record paper at a pixel density of 16 pixels/mm, and the dots are formed in parallel in a lateral direction of the record paper and the record paper is subscanned longitudinally at a speed of 3 second/sheet, it is necessary to print 6720 lines in three seconds. Because 4800 dot forming elements are included in each line, 32,256,000 pixel data are to be transferred in three seconds. It corresponds to a data transmission rate of 10.75 M bps. As described above, since the image data transfer time to the shift register and the recirculation time for the dot formation are separated from each other and a switching time is required between line outputs, a transfer rate of 15 Mbps-20 Mbps is required to output the image data in three seconds.
In a prior art multi-level printer, if the number of tonalities is 17 and a bit length of the shift register is 16, a 16-bit data must be transferred for each pixel. Assuming that this multi-level printer is operated under the same condition as that described above, that is, at a print speed of 3 seconds/sheet and three seconds are required to transfer the data, the transfer rate is as high as 172 Mbps which is too high to attain.
An existing electro-photographic copier, which is widely used has a printing capability of 20 sheets/minute for size A3 paper. When a digital copier having a digital reader and a digital printer is used, a print capability of 15-20 sheets per minute is desired. Accordingly, a high speed image output device which reproduces image within three seconds per record sheet of size A3 is strongly desired.